1. Field of the Invention
The present invention is directed to a system for preparing a human patella to receive a prosthesis and, thereafter, for implanting the prosthesis in the patella.
2. Description of the Prior Art
In total knee replacement surgery, a prosthesis is provided in which one component is fastened to the distal end of the femur which has been resectioned and another component is fastened to the proximal end of the tibia which has been resectioned so that the two components will act together in permitting the leg a full range of motion. In performing such surgery, the patella is not completely replaced but rather is resectioned such that the posterior dome portion facing the condyles is cut and reamed to form a cavity in which a patellar prosthesis is implanted. The patellar prosthesis has a dome facing inwardly to engage the patellar or trochlear groove of the femoral component.
Heretofore, in preparing a patella for an implant, it was necessary to firmly clamp the patella, perform a free hand resection using an oscillating saw, then ream a cavity in the patella of sufficient size to receive the patellar implant intended to be used. Depending upon a number of factors including the size of the patient, the patellar implant could be one of a number of different sizes. As a result of this, it was necessary to have available a number of guides, each having a different diameter, for guiding various size reamers. Typical prior art methods of forming a cavity for implanting a patellar prosthesis are shown and described in the following publication, which is incorporated herein by reference: a brochure entitled "Legend II Surgical Technique--The Concept of Personalization" copyright 1992, DePuy, Inc., Warsaw, Ind. A copy of this reference accompanies this disclosure.
Typically, a surgeon will select to install a patellar prosthesis by utilizing either (a) a resurfacing technique wherein the prosthetic patella will be positioned on the prepared surface of the patella or (b) an insetting technique wherein the prosthetic patella will be recessed or inset into the prepared surface of the patella. Accordingly, two varieties of guides could be available for connection to the patellar clamp and could be interchangeably connectable to the clamp. The interchangeable guides permit a surgeon to use the patellar clamp during either a total patellar bone surfacing procedure or a patella insetting procedure.
In either a total resurfacing procedure or a patella insetting procedure, it is important that a sufficient amount of bone stock remain after resection to accept the fixation means of the patellar prosthesis and maintain the integrity of the remaining patellar bone. If too much bone material is removed, the remaining patellar bone may be structurally weak.
In order to prevent an excessive amount of bone material from being resected, a prior art clamp and reamer included a stop ring carried by the shaft above the reamer blades. This type of stop controls the amount of bone removed from the patella, but does not control the minimum thickness of patella bone remaining. The stop ring engages with the upper surface of the reamer guide to stop the shaft and reamer from being further advanced into the patellar bone. A plurality of stop rings having a plurality of effective thicknesses may be interchangeably connected to the shaft of the reamer to accommodate the varying amount of bone to be resected as the thickness of the patellar bone varies between patients. The stop rings may be calibrated and may include measurement indicia thereon, in terms of the amount of bone material to be removed during resection. For example, if a surgeon needs to resect 10 mm of bone material, a ringed stop having the indication "10 mm", and correspondingly an effective thickness of 10 mm, would be turned onto the shaft. Therefore, the shaft and reamer would only be able to resect 10 mm of bone material before the stop ring engages the guide.
In a more recent development disclosed in U.S. Pat. No. 5,129,907 to Heldreth et al., an adjustable depth gauge wing is carried by the clamp and engaged by a collar on the reamer shaft. In this construction, the engagement of the collar and depth gauge wing prevents further movement of the reamer into the patella. The adjustable depth gauge wing includes a stepped upper surface having measurement indicia thereon which refer to distance in terms of the amount of patellar bone material to be left after resection. Four stepped levels are provided at 2 mm increments.
U.S. Pat. No. 4,964,867 to Boger and commonly assigned discloses a patellar prosthesis provided with a pair of concave protuberances for reception in an annular groove in a prepared resection of a natural patella. The instant invention might well utilize a prosthesis generally of this construction.
Another recent development, also commonly assigned, although not one specifically related to patellar prostheses is disclosed in U.S. Pat. No. 4,710,075 to Davison. The patented construction relates to a bone drill which provides a drill stop collar which can be selectively moved to and retained at a desired position along the length of a drill bit. When the stop collar engages the surface of the bone, axial movement of the drill bit ceases.